1. Field of the Invention
This invention relates to method of bleaching paper pulp, especially recycled paper pulp which has been subjected to de-inking, to increase brightness and reduce the content of chromophores.
The process of de-inking paper pulp, for example recycled newsprint, involves the use of alkalis such as sodium hydroxide which tend to cause yellowing of the pulp. There is therefore a need for a subsequent bleaching
2. Description of the Prior Art
One type of reductive bleaching process for this purpose is disclosed in FR-A-2639371, according to which de-inked pulp is bleached by the combined action of:
a) sulphite ions derived from sodium sulphite, sodium bisulphite, a combination of the two or sulphur dioxide and sodium hydroxide and PA1 b) a reducing agent more electronegative than sulphite ions, for example thiourea dioxide, sodium borohydride or sodium dithionite.
Where sodium sulphite and/or bisulphite and sodium borohydride are used together, sodium dithionite is generated in situ to act as a reducing agent.